Should charities charge for their services or be free?

My husband works with a couple medical missions groups that travels to poorer countries like Haiti. They go to perform health care including small surgeries. The group firmly believes in charging the patients. Obviously they don't charge much, it's like the equivalent of $5 to us. But they believe if they don't charge then no one would actually value their service and would also become dependent on outside help. They think it also teaches them to save for things they value. I don't think they would actually turn anyone away but so far everyone has been able to pay the fees. For the record, the group doesn't actually make any money at all. Everyone pays their own way to go. The money they do make just pays for supplies. What do you think? Are people less likely to value and respect a service if it's at no cost to them? Or is it wrong to charge the poor for services?